A polymer electrolyte fuel cell (hereinafter referred to as “PEFC”) causes a hydrogen-containing fuel gas and an oxygen-containing oxidizing gas, such as air, to electrochemically react with each other to generate electric power and heat at the same time. A unit cell (cell) of the PEFC includes: an MEA (Membrane-Electrode-Assembly; a polymer electrolyte layer-electrode stack body) constituted by a polymer electrolyte layer and a pair of gas diffusion electrodes; gaskets; and electrically-conductive separators. A groove-like gas channel through which the fuel gas or the oxidizing gas (each of which is hereinafter referred to as “reactant gas”) is formed on a main surface of each separator which surface contacts the gas diffusion electrode. The gaskets are disposed around a peripheral portion of the MEA, and the pair of separators sandwich the MEA. Thus, the cell is formed. A common cell stack is so-called a stack-type cell stack in which the cells are stacked on one another and fastened, and adjacent MEAs are electrically connected to each other in series.
The cell stack configured as above utilizes the heat generated by electric power generation to maintain the cell stack itself at high temperature, thereby improving an electric power generation efficiency. Further, for example, a domestic fuel cell cogeneration system boils water using remaining heat energy, thereby improving an energy utilization rate. Moreover, Patent Document 1 discloses a polymer electrolyte fuel cell in which a heat insulating material covers an outer surface of the cell stack to further effectively utilize the heat energy generated by the electric power generation.
As described above, the cell stack is a stack of cells. Therefore, to detect abnormalities of the cell stack, it is necessary to monitor not only the voltage of the entire cell stack but also the voltage of the electric power generated by each cell.
However, it has conventionally been difficult to simply and surely measure the voltage of each cell. This is because each cell is reduced in thickness to reduce the weight of the cell stack and downsize the cell stack, so that to measure the voltage of each cell without short-circuiting adjacent cells, a voltage measuring terminal of a voltage measuring device needs to be surely pressed against the target cell, and this requires accurate operations and high dimensional accuracy. In addition, measuring the voltage of each cell is difficult because since the cell stack is formed by stacking a large number of cells, the voltage measuring terminals need to be attached to all the cells, and this requires a large number of operation steps.
To solve the above problems, a fuel cell monitor is known, in which a plurality of voltage measuring terminals are integrated with a comb-teeth support plate (see Patent Document 2 for example). In accordance with the fuel cell monitor disclosed in Patent Document 2, since the voltage measuring terminals are supported by the comb teeth of the support plate, the voltage measuring terminals for a plurality of cells can be attached to the cell stack at one time, and this attaching operation is simple. Moreover, known as a fuel cell stack which simplifies an operation of attaching a voltage detecting device is a fuel cell stack in which a cell voltage terminal is integrated with the separator (see Patent Document 3 for example).
Moreover, known as a voltage measuring device which surely causes the voltage measuring terminal to contact the separator is a voltage measuring device in which an electrical insulating elastic plate is provided with a voltage obtaining terminal (see Patent Document 4 for example). In the voltage measuring device disclosed in Patent Document 4, the separator is provided with a concave portion, and the voltage obtaining terminal is pressed against the concave portion by the elastic force of the elastic plate. With this, the voltage obtaining terminal can be surely caused to contact the separator, and the accuracy of the voltage measurement can be improved.
Further, known as a fuel cell stack which effectively maintain the stiffness of a casing itself is a fuel cell stack in which an opening is formed for every unit cell or every plural unit cells on at least one surface of the casing (see Patent Document 5 for example). In the fuel cell stack disclosed in Patent Document 5, a plurality of openings each larger than the voltage terminal are formed at positions corresponding to the voltage terminals each attached for every unit cell or every plural unit cells. This can effectively maintain the stiffness of the casing as compared to a case of forming an elongated opening extending in a stack direction of the unit cells.    Patent Document 1: Japanese Laid-Open Patent Application Publication 2005-327558    Patent Document 2: Japanese Laid-Open Patent Application Publication 2006-140166    Patent Document 3: Japanese Laid-Open Patent Application Publication 2005-216700    Patent Document 4: Japanese Laid-Open Patent Application Publication 2004-362860    Patent Document 5: Japanese Laid-Open Patent Application Publication